For sheets, if you're happy with the length take the old ones off determine the gauge and and match them up. They might have swollen, so the new will appear to be a smaller gauge. A good boat supply store will be able to help.
For halyards you can do the same, you will need to attach a messenger line to the end of the halyards pull the old ones out and make sure you tie the messengers off good at both ends!
For the messenger, I think I just used a heavy gauge coated carpet thread..
When you pull the new halyards through, I sewed the end of the messenger to the new halyard and then taped it with a good overlap
For everything else..outhauls, downhauls, topping lifts, etc..the process is similar. This is not the only way...it's a way..when you don't know what you have...once you do this, write all the lengths, gauges and type that you purchase and keep it in your log
New England Rope has a handy dandy selector guide for their products
book.New England Ropes - Line Selector
A good marina store can help you.